This invention relates to continuously variable transmissions (CVT) with a geared neutral condition.
Continuously variable transmissions generally employ a continuously variable unit (CVU) such as a belt and sheave mechanism, electric motor/generator systems, or hydraulic pump/motor systems. The electrical and hydraulic units can achieve a neutral condition by simply not supplying energy to the drive unit (i.e. the motor). Belt and sheave mechanisms however, must incorporate either a clutch mechanism or a summing differential gearing assembly that will permit the output speed to be zero while the input speed is not zero.
One such unit can be found in U.S. Pat. No. 4,644,820 issued to Macey and Vahabzadeh (Macey et al.) on Feb. 24, 1987. This patent incorporates two selectively engageable friction clutches and one one-way clutch, in the mechanical power path, between the input shaft, driven by a prime mover, and an input member of the summing differential gearing assembly. The CVU output is continuously connected with another input member of the summing differential gearing assembly. To achieve a high ratio drive through the CVU, the input clutches in the mechanical power path must be disengaged.
The Macey et al. patent does not permit a mechanical high range drive condition. Thus, the efficiency loss of the CVU is always present during the operation of the CVT. The incorporation of two friction clutches and a one-way clutch adds to the complexity of the transmission without additional benefit from the mechanical power path. Also this patent requires a total of three selectively engageable clutches and a one-way clutch to establish a low range and a high range of operation.
It is an object of the present invention to provide an improved continuously variable transmission.
In one aspect of the present invention, a variable ratio belt drive path, a direct mechanical drive path, and a planetary gear set are disposed in a power path between an input shaft and an output shaft to provide a geared neutral condition, a forward continuously variable range and a reverse continuously variable range. In another aspect of the present invention, the planetary gear set operates as a summing differential gearing assembly or unit during a low forward range and a reverse range and is bypassed during a continuously variable high forward range during which only the variable ratio belt drive path is active.
In yet another aspect of the present invention, a discrete mechanical path is provided between the input shaft and the output shaft in bypassing relation with the summing differential gearing assembly. In still another aspect of the present invention, the discrete mechanical path provides a discrete ratio at a point within the continuously variable high forward range or at the upper end of the continuously variable high forward range to thereby establish an efficient operating point in the power path.